High tensile-strength galvanized steel sheets for use in automobile parts and the like must have excellent formability as well as a high strength because of the characteristics of the applications.
Recently, high tensile-strength steel sheets have been required and increasingly used as materials for automobile bodies to improve fuel efficiency by weight reduction and ensure crashworthiness. Furthermore, while high tensile-strength steel sheets have mainly been used in simple processing applications, they are also being applied to complicated shapes..
However, in general, higher-strength steel sheets tend to have lower formability. In particular, the most important problem in the application of high tensile-strength steel sheets is cracks in press forming. Thus, formability, such as stretch flangeability, must be improved in a manner that depends on the shape of a part. In particular, high tensile-strength steel sheets having a TS of at least 980 MPa are often used in parts that are to be bent. Thus, bendability (synonymous with bending formability) is also important.
Furthermore, after forming of a steel sheet, the steel sheet is subjected to resistance spot welding in an assembly process. Thus, in addition to formability, excellent weldability is also required.
To this end, for example, Japanese Unexamined Patent Application Publications No. 2004-232011, No. 2002-256386, No. 2002-317245, and No. 2005-105367, Japanese Patent No. 3263143 and its Japanese Unexamined Patent Application Publication No. 6-073497, Japanese Patent No. 3596316 and its Japanese Unexamined Patent Application Publication No. 11-236621, and Japanese Unexamined Patent Application Publications No. 2001-11538 and No. 2006-63360 propose a method for manufacturing a high tensile-strength galvanized steel sheet having excellent formability, for example, by defining the steel component and the microstructure or by optimizing hot-rolling conditions or annealing conditions.
Among the publications described above, Japanese Unexamined Patent Application Publication No. 2004-232011 discloses steel having high C and Si contents and of TS 980 MPa grade. However, excellent stretch flangeability or bendability is not the primary objective of Japanese Unexamined Patent Application Publication No. 2004-232011. Furthermore, exemplified compositions have poor platability (require iron-based preplating), and resistance spot weldability is also difficult to achieve.
Japanese Unexamined Patent Application Publication Nos. 2002-256386, 2002-317245 and 2005-105367 disclose steel leveraging Cr. However, excellent stretch flangeability and bendability is not the primary objective of these publications. Furthermore, it is difficult to achieve a TS of at least 980 MPa by these techniques without the addition of a strengthening element in such an amount that the characteristics described above or platability is adversely affected.
Furthermore, Japanese Patent No. 3263143 and its Japanese Unexamined Patent Application Publication No. 6-073497, Japanese Patent No. 3596316 and its Japanese Unexamined Patent Application Publication No. 11-236621 and Japanese Unexamined Patent Application Publication No. 2001-11538 describe a hole expansion ratio λ, which is an indicator of stretch flangeability, but rarely achieve a tensile strength (TS) of 980 MPa. The tensile strength (TS) of 980 MPa is only achieved in Japanese Patent No. 3596316 by the addition of large amounts of C and Al, which is unfavorable to resistance spot weldability. Furthermore, excellent bendability is not the primary objective of Japanese Patent No. 3596316.
Japanese Unexamined Patent Application Publication No. 2006-63360 describes a technique in which bendability or fatigue characteristics are improved by the addition of Ti. However, excellent stretch flangeability or weldability is not the primary objective of Japanese Unexamined Patent Application Publication No. 2006-63360.
In view of the situations described above, it could be helpful to provide a high tensile-strength galvanized steel sheet that has a tensile strength as high as 980 MPa or more and excellent formability and weldability, as well as excellent bendability. It could also be helpful to provide an advantageous method for manufacturing the high tensile-strength galvanized steel sheet.